1. Field of the Invention
This invention relates to the use of metal ammine complexes for storage of ammonia in solid form and for systems utilizing the solid storage material for delivery of ammonia by release of ammonia from the material using electromagnetic radiation. Upon release, ammonia may be used as the reducing agent in selective catalytic reduction (SCR) of NOx in exhaust gases from combustion processes.
Other applications using ammonia in mobile or portable units or in special chemical synthesis routes where storage of liquid ammonia is too hazardous are also considered embodiments of the present invention.
2. Description of the Related Art
Current environmental regulations necessitate the use of catalysts in the treatment of exhaust gas from automotive vehicles, boilers and furnaces for control of NOx emissions. Particularly, vehicles equipped with diesel or other lean burn (gasoline) engines offer the benefit of improved fuel economy, but catalytic reduction of NOx using conventional car exhaust catalysts (three-way catalyst) is not feasible because of the high oxygen content in the exhaust gas. Instead, selective catalytic reduction (SCR) has proven useful for achieving the required low levels of NOx in the exhaust gas both in stationary and mobile units. In such systems NOx is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NOx.
So far, the most efficient reductant has been ammonia, which is usually introduced into the exhaust gas by controlled injection either of gaseous ammonia, aqueous ammonia or indirectly as urea dissolved in water. In all cases, the amount of reductant being dosed has to be very precisely controlled. Injection of too high amount of reductant will cause a slip of ammonia in the exhaust gas whereas injection of a too small amount of reductant causes a less than optimal conversion of NOx.
In many mobile units, the available technology is to use an aqueous solution of urea as the reductant since in this way potential hazards or safety issues relating to the transport of liquid ammonia are eliminated. However, there are several disadvantages related to the use of aqueous urea as the reductant. First of all, the use of urea solutions requires that a relatively large storage volume is available in order to enable transport sufficient amounts of ammonia. In typical systems, about 30 wt % urea solution is preferred meaning that about 70 wt % of a container holding the urea solution is used only to transport water. During operation the urea solution is sprayed into the exhaust gas, the droplets evaporate and the urea decomposes to ammonia (one molecule of urea forms two molecules of NH3 and one CO2) which by mass is roughly 50 wt % of ammonia in the urea molecule. Similar concentrations of ammonia can be achieved using aqueous solutions of ammonia as reductant. Furthermore, for technologies using aqueous solutions a specially designed spray nozzle combined with a precision liquid pump is required to ensure that a) the aqueous urea is delivered to the exhaust system at a desired (and dynamically changing) flow rate and b) that the solution is efficiently dispersed as fine droplets in the gas phase before entering the catalyst. Furthermore, the aqueous solutions might freeze in extreme weather conditions, or the urea solution may simply form precipitates, which might block the dosing system, e.g. the nozzle. Therefore, all lines have to be heated. Furthermore, the decomposition of urea may not proceed as wanted. There may be undesired side-reactions giving by-products in the form of solid deposits of polymers (melamine) and these side reactions also make it difficult to dose a very specific amount of ammonia since the amount of free ammonia released from a given amount of urea can vary according to the decomposition conditions.
Altogether, these difficulties may limit the possibilities of using SCR technology in pollution abatement, particularly in connection with mobile units. To circumvent these difficulties, the present invention devises an alternative method for transporting and dosing ammonia to exhaust gases prior to entering SCR catalyst systems.
As disclosed in applicant's copending application No. PCT/DK 2005/00516 metal ammine salts can be used as a solid storage media for ammonia which in turn may be used as the reductant in selective catalytic reduction to reduce NOx emissions from automotive vehicles, boilers and furnaces. Thus, the metal-ammine salt constitutes a solid storage medium for ammonia, which represent a safe and practical option for storage and transportation of ammonia. Usually, ammonia is released thermally from the preferred metal ammine salt by external heating, see e.g. European Patent No. EP 0 932 440 B1. The metal ammine salt is held in a container from which the released ammonia is dosed through a controllable valve directly into the exhaust gas in the desired proportion. Between the container and the valve, there may be a small buffer volume to increase the controllability of the system. Useful metal ammine salts have the general formula M(NH3)nXz, where M2+ is one or more metal ions capable of binding ammonia (For example M may be Li, Mg, Ca, Sr, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc.), n is the coordination number (2-12), and X is one or more anions, where representative examples of X are F, Cl, Br, I, SO4, MoO4, PO4 etc.
During release of ammonia, the original metal-ammine salt M(NH3)nXz is gradually transformed into M(NH3)mXz with m<n. When all the desired ammonia has been released, the resulting M(NH3)mXz can usually be converted back into M(NH3)nXz by an absorption treatment with an ammonia-containing gas stream.
Typical ammonia contents of the metal ammine complexes are in the range of 20-60 wt %, preferably above 30 wt %. As an example, a typical and inexpensive compound such as Mg(NH3)6Cl2 contains 51.7 wt % ammonia. Using a compaction method such as the one disclosed in applicant's copending application No. PCT/DK 2006/00059 it is possible to obtain an ammonia density per unit volume above 90% of that of liquid ammonia.
Using applicant's technology enables storage of ammonia at significantly higher densities (both on a volume and a weight basis) than both aqueous ammonia and aqueous urea solutions. For several metal ammine salts it is possible to release all ammonia and then transform the resulting material back into the original metal ammine salt in a large number of cycles. This obviously constitutes preferred embodiments. Additionally, the ammonia is directly delivered in the form of a gas, which is an advantage in itself—both for the simplicity of the flow control system and for an efficient mixing of reducing agent, ammonia, with the exhaust gas—but it also eliminates potential difficulties related to blocking of the dosing system because of precipitation or impurities in a liquid-based system.
For mobile units, it is particularly useful to hold the metal ammine in a container that can be easily separated from mobile unit and replaced by a new metal ammine container. In preferred embodiments, the metal ammine containers are recycled and recharged with ammonia in a separate recharging unit or recharging facility.
Usually ammonia is released by normal heating generated by electrical resistance in heating elements or by using the heat from the exhaust gas. It is easy but poses several drawbacks: Since heat is supplied from an external source both ammonia depleted salt close to the heating element, the container itself as well as the saturated salt are also heated. Only the heating of the saturated salt results in additional release of ammonia and the heat absorbed in the container and the unsaturated salt is in principle wasted. Especially, under non-steady state operation this energy is lost during each start and stop cycle. Further, the response time of the system is limited since heat has to propagate by normal heat conduction from the heating element through unsaturated (depleted) salt to the saturated salt. This time response can be long making it more difficult to control the ammonia release and extending the duration of start-up. This is very important for emission control in the first 5-10 minutes after start up of an engine. Therefore, new methods and devices for efficient release of ammonia from the solid material in the container are attractive in this technical field.